Scanning probe microscopic imaging of guanine on a highly oriented pyrolytic graphite electrode.

Guanine adsorbed onto a highly oriented pyrolytic graphite electrode was studied by MAC-Mode Atomic Force Microscopy (AFM), and the electrochemical behaviour of the guanine layer was investigated with Electrochemical AFM. Guanine adsorbs spontaneously, without forming a well-packed structure, into nucleation spots, which are stable with time and cover the surface uniformly and almost completely. The process of guanine adsorption and nucleation can be controlled and the effect of altering the exposure time and varying the potential was investigated.     

Development of an HPLC method with electrochemical detection of femtomoles of 8-oxo-7,8-dihydroguanine and 8-oxo-7,8-dihydro-2'-deoxyguanosine in the presence of uric acid

A selective method based on high performance liquid chromatography with electrochemical detection (HPLC-ECD) was developed to enable simultaneous detection of 8-oxo-7,8-dihydroguanine (8-oxoGua) and 8-oxo-7,8-dihydro-2′-deoxyguanosine (8-oxodGuo), products of DNA oxidative damage, in the presence of uric acid (UA), a strong interferent in their electrochemical detection. The method developed consists of HPLC isocratic elution with amperometric detection on a glassy carbon electrode, enabling a detection limit for 8-oxoGua and 8-oxodGuo lower than 1 nM in standard mixtures. Detection of low concentrations up to 25 nM of 8-oxoGua and 8-oxodGuo in the presence of UA in a 10⁴-fold higher concentration was achieved after one-step solid phase extraction (SPE). The method was tested with urine samples and it was possible to detect and quantify the presence of 8-oxoGua, and to confirm that UA was eliminated after uricase degradation and SPE. The LOD found in urine samples was about 80 nM, a value higher than in standard mixtures, due to the increase of background current in the urine matrix. The results presented here contribute to the development of a methodological approach to simultaneous determination of 8-oxoGua and 8-oxodGuo in urine samples.     

(Z)-3-p-tolylsulfinylacrylonitrile as a chiral dienophile: diels-alder reactions with furan and acyclic dienes

The behavior of (Z)-3-p-tolylsulfinylacrylonitrile (1) as a chiral dienophile has been evaluated from its reactions with furan and acyclic dienes. Electrostatic interactions of the cyano group with the sulfinyl one restrict the conformational mobility around the C-S bond, thus controlling the pi-facial selectivity, which is almost complete in all cases, the approach of the diene from the less-hindered face of the dienophile (that bearing the lone electron pair) in the predominant rotamer being the favored one. The regioselectivity is also completely controlled by the cyano group. Additionally, the reactivity of compound 1 as well as its endo-selectivity are both higher than those observed for the corresponding (Z)-3-sulfinylacrylates, thus proving the potential of sulfinylnitriles as chiral dienophiles.     

Narrow band quantitative and multivariate electroencephalogram analysis of peri-adolescent period

ABSTRACT: BACKGROUND: The peri-adolescent period is a crucial developmental moment of transition from childhood to emergent adulthood. The present report analyses the differences in Power Spectrum (PS) of the Electroencephalogram (EEG) between late childhood (24 children between 8 and 13 years old) and young adulthood (24 young adults between 18 and 23 years old). RESULTS: The narrow band analysis of the Electroencephalogram was computed in the frequency range of 0--20 Hz. The analysis of mean and variance suggested that six frequency ranges presented a different rate of maturation at these ages, namely: low delta, delta-theta, low alpha, high alpha, low beta and high beta. For most of these bands the maturation seems to occur later in anterior sites than posterior sites. Correlational analysis showed a lower pattern of correlation between different frequencies in children than in young adults, suggesting a certain asynchrony in the maturation of different rhythms. The topographical analysis revealed similar topographies of the different rhythms in children and young adults. Principal Component Analysis (PCA) demonstrated the same internal structure for the Electroencephalogram of both age groups. Principal Component Analysis allowed to separate four subcomponents in the alpha range. All these subcomponents peaked at a lower frequency in children than in young adults. CONCLUSIONS: The present approaches complement and solve some of the incertitudes when the classical brain broad rhythm analysis is applied. Children have a higher absolute power than young adults for frequency ranges between 0-20 Hz, the correlation of Power Spectrum (PS) with age and the variance age comparison showed that there are six ranges of frequencies that can distinguish the level of EEG maturation in children and adults. The establishment of maturational order of different frequencies and its possible maturational interdependence would require a complete series including all the different ages.     

An impedance study of the adsorption of nucleic acid bases at glassy carbon electrodes

Polyamidoamine (PAMAM) dendrimers (generation 3.5 and 4) interaction with bovine serum albumin (BSA) was studied. The intensity of intrinsic fluorescence of two tryptophan residues and a shift in wavelength of their emission maxima were chosen as indicators of protein conformational changes. It is shown that the generation 4 has a greater impact on spectral properties of serum albumin than generation 3.5.     

A DNA-electrochemical biosensor for screening environmental damage caused by s-triazine derivatives

An electrochemical DNA-biosensor has been used to investigate the interactions between DNA and members of a group of ten derivatives of 1,3,5-triazine herbicides: chloro-s-triazines (atrazine, propazine, terbutylazin, and cyanazin), thiomethyl-s-triazines (ametryn, prometryn, terbutryn, and simetryn), and methoxy-s-triazines (prometon and terbumeton). A UV spectrophotometric study of this group of herbicides was also undertaken. Of this group only cyanazin could be oxidized in aqueous solution using a glassy carbon electrode. Use of the electrochemical DNA-biosensor revealed the occurrence of a time-dependent interaction of all the herbicides with DNA, via the appearance of guanine, guanosine, and adenosine oxidation signals that correspond to DNA damage. Adduct formation between the herbicide and the DNA purine bases guanine and adenine is suggested as a mechanism.     

Electrochemical study of quercetin-DNA interactions: part I. Analysis in incubated solutions

The present study aims to investigate the quercetin-deoxyribonucleic acid (DNA) interaction occurring in bulk solution either electrochemically using differential pulse voltammetry or spectrophotometrically, in order to explain the possible DNA-damaging activity of quercetin. A very weak interaction between quercetin and DNA in solution was found to take place. However, since extensive quercetin-induced DNA damage via reaction with Cu(II) has been reported, an electrochemical study of the DNA-Cu(II)-quercetin system in solution was undertaken. The product of DNA interaction with quercetin-Cu(II) complex was observed. Damages to DNA were electrochemically recognized via the increasing of the anodic peaks corresponding to the oxidation of guanosine and adenosine bases and spectrophotometrically via increasing of the 260 nm adsorption band. It was also observed that dsDNA damage produced by the quercetin-Cu(II) complex occurred with time. Control experiments with different mixtures of Cu(II), quercetin, ssDNA, dsDNA or poly[A] were carried out in order to establish a possible mechanism of interaction between DNA and quercetin via Cu(II).     

Scars of the Wigner Function

We propose a picture of Wigner function scars as a sequence of concentric rings along a two-dimensional surface inside a periodic orbit. This is verified for a two-dimensional plane that contains a classical hyperbolic orbit of a Hamiltonian system with 2 degrees of freedom. The stationary wave functions are the familiar mixture of scarred and random waves, but the spectral average of the Wigner functions in part of the plane is nearly that of a harmonic oscillator and individual states are also remarkably regular. These results are interpreted in terms of the semiclassical picture of chords and centers.